Why Do We Forget How to Walk on Ice?

A new book explores the crosstalk between mind and body and how it helps humans navigate their worlds.

Scott Grafton

Jan 13, 2020

When people ask me about the “mind-body connection,” I typically suggest walking on an icy sidewalk. Skip the yoga, mindfulness, or meditation, and head to the corner on a cold, windy, snowy day. Every winter, much of North America becomes exceedingly slippery with ice. Emergency rooms across the continent see a sharp uptick in fractured limbs and hips as people confidently trudge outside in such conditions, unveiling a profound disconnection between what people believe and what they can actually do with their bodies. One might think that a person could call on experience from years past to adjust their movement or provide a little insight or caution. But the truth is that the body forgets what it takes to stay upright in these perilous conditions. Why is there so much forgetting and relearning on an annual basis? We remember how to ride a bike. Why can’t we remember how to walk on ice?

Falling on ice reveals a delicate tradeoff that the brain must reconcile as it pilots the body. On the one hand, it needs to build refined motor programs to execute skills such as walking, running, and throwing. On the other hand, those programs can’t be too specific. There is a constant need to tweak motor plans to account for dynamic conditions. When I throw a backpack on, my legs don’t walk in the same way as they do without the pack: my stance widens, my stride shortens. Often, the tweaking needs to happen in moments. As I pick the pack up, I need to lean in or I could tip myself over. Just as importantly, as soon as I put it down, I need to forget I ever held it in the first place.

In my lab at the University of California, Santa Barbara, we investigate the mechanisms the brain employs to make these super-fast adaptations. We have participants pick up tippy objects the size of a desk stapler that are weighted to roll to the left or right. After only a few trials with one object, subjects typically succeed without any mistakes. Then, we switch the object. The tipping is back, but in the opposite direction. The subjects learn yet again.

We know from neurophysiologic recordings and brain imaging that the skills necessary to pick up objects are controlled by motor areas of the cortex, the brain stem, and the spinal cord. From fMRI scans we know the cerebellum is a key area for acquiring and forgetting these rapid adaptations of finger forces. Pattern analysis of the brain scans suggests that when a participant must learn how to successfully lift a new object, the brain can completely erase the pattern previously used to lift the one that rolled in the opposite direction. The brain falls back to a generic pattern for lift forces that it applies whenever it has to refamiliarize itself with the dynamic properties of the object. Fast adaptation like this occurs whenever a person’s body is in motion, whether it is an adjustment of the fingers to pick up an object or of the legs to walk down a slippery sidewalk.

We are usually unaware of the countless small tweaks we make to our movement patterns as we adapt to changing conditions. It is one of the many aspects of physical intelligence that we take for granted. A lack of insight into the forgetting in particular is one explanation for why people fall on ice. As winter sets in, the tweaks necessary to stay upright on the ice are long since forgotten. We simply don’t retain this kind of dynamic knowledge. The only way to adapt to a slippery surface is to experience it anew through direct contact. A marathon of virtual reality exposure wouldn’t help a bit. Only by shuffling, sliding, and feeling the grip of our shoes can we adapt and learn what is physically possible in those specific conditions. As this crosstalk between mind and body helps us attain equilibrium, beliefs and motions are brought into harmony. And the ice walker trods steadily on.

Scott Grafton is a neuroscientist at the University of California, Santa Barbara. Read an excerpt of Physical Intelligence at the-scientist.com.